The invention relates to the field of automatic transmission control, particularly to a control of a forward drive clutch or a reverse drive clutch in a vehicle driveline that does not include a hydrokinetic torque converter.
In conventional multiple-speed transmissions, a hydrokinetic torque converter is located in the driveline between the crankshaft of an internal combustion engine and multiple speed gearing, which establishes torque flow paths to the vehicle traction wheels. Attempts have been made to eliminate the torque converter from the driveline in order to eliminate its low hydrokinetic efficiency. Eliminating the torque converter also reduces the rotary mass of the torque transfer elements at the input side of the gearing.
The driveline of a conventional motor vehicle is a very lightly damped second order mechanical system having of two elements—inertia and torsional stiffness. Transient disturbances to this system, such as those that result by rapidly depressing the accelerator pedal (a “tip-in”) or suddenly releasing the pedal (a “tip-out”), can cause vibratory oscillations of the system.
The gears, splines and chain drives in an automatic transmission always have some lash, the clearance between connected mechanical components. The effect of lash is most apparent in a driveline when the net torque passes through zero between positive and negative torque, such as when a tip-in or tip-out occurs. During a tip-in, an increase in input torque accelerates the system inertia. Normally the driveline stiffness helps to resist acceleration of the inertia. When clearance or lash is present, however, system inertia is able to accelerate freely until lash is taken up by displacement of driveline components. When lash is taken up, the extra energy stored in the system inertia is suddenly placed on the driveline, creating an impulse. This impulse of energy released into the driveline causes an oscillation in the system. Under these conditions, there is very little damping capacity in the driveline to attenuate vibrations.
In a conventional automatic transmission, the torque converter provides a passive hydraulic coupling between the driveline and the engine, which helps to reduce transient vibrations. Even with a torque converter, a tip-in or tip-out is a major concern when the input torque reverses direction from a positive torque condition to a negative torque condition. For a tip-out, many manufacturers force the engine to continue producing positive torque by actively controlling an idle air bypass system so that the input torque does not reach zero.
In a pre-transmission hybrid powertrain, such as that described in U.S. Pat. No. 6,176,808, driveline transient torque impulses are difficult to attenuate because there is no torque converter in the driveline to provide a source of damping, and a hybrid vehicle uses brake regeneration to recover much of the braking energy from the vehicle. Brake regeneration imposes a negative torque on the driveline, which the transmission must transfer whenever the accelerator pedal is off. Because the transmission must transfer negative torque, idle air bypass control is not available to maintain positive driveline torque.
U.S. Pat. No. 6,299,565 describes a solution to this problem for a powershift transmission. Slip control of the input clutch was accomplished along with control of the input through filtering of the engine throttle. This solution required Electronic Throttle Control (ETC) to slow the change in input similar to using idle air bypass control for a tip-out.
There is need for slip control of the input clutch to deal with torsional impulses in a driveline whose engine has a mechanical throttle, without using ETC to manipulate idle air.